1. Field of the Invention
This invention relates to a positioning technique. In particular, it relates to a method and apparatus for positioning a mobile station in a Time-Division Multiple Access (TDMA) communication system.
2. Description of the Related Art
Wireless telephony has become a popular form of communication in recent years. Furthermore, the value of positioning services, whereby the location of a user is pinpointed, has been recognized. Thus, using the same personal communication equipment as the platform for voice/data communication and positioning service would be both convenient and cost-saving. The positioning service is provided using the existing communication system, wherein a wireless handset is used for communication and serving as the signal source to be located.
Outdoor positioning techniques have been well developed. For example, the Global Positioning System (GPS) and multiple-base-station system utilize GPS satellites and base stations, respectively, to determine a user's position. However, these techniques have their limitations in the indoor application. For example, for the GPS technique, the GPS signal has difficulty penetrating an indoor wall. For the multiple-base-station technique, a user may not be able to see 3 or more base stations.
Several solutions have been proposed to provide the solutions of indoor positioning. As disclosed in U.S. Pat. No. 5,604,765, a signal similar to the GPS spread spectrum is incorporated in a handset or other personal communication equipment. However, using this technique, it is necessary to use a general GPS receiver to decode the signal which incurs additional cost. Another method is to increase the number of base stations to enhance the probability that the handset signal will be received, but adds substantial cost. Another way is to use a distributed antenna technique wherein the antenna is applied as a relay located between the base station and the mobile station (for example, a handset), thereby extending the coverage area of the antenna. However, this is more appropriate to a Code-Division Multiple Access (CDMA) system or spread spectrum system. It does not work well in the TDMA system because of the signal multipath phenomenon, which causes a severe intersymbol interference (ISI).
The 3D-ID technique developed by Pinpoint Corporation utilizes the GPS concept. In place of GPS positioning satellites, it utilizes mounted indoor nodes to function as the GPS satellite. A specific unit such as a handset receives a spread spectrum signal from each node and transforms the received signal into another frequency, which it sends out the changed frequency signal with an identifier in a Phase Modulation (PM) manner. After receiving these returned signals from the specific unit, the system controller determines the position of the specific unit by three point positioning technique. However, this method requires setting up a large number of GPS nodes. Further, these nodes not only have to receive and transmit the signal, but also process the modulation data in the signal. Therefore, the cost of the nodes is high.